The manufacture of wafers, such as silicon wafers, involves a number of sequential steps to produce a wafer that meets that exacting specifications of the various device manufacturers. Initially, a crystalline ingot is grown, such as by the Czochralski method. The crystalline ingot is sliced into a plurality of wafers. The edge of each wafer is then generally ground to properly size the wafer and to impart the desired profile, such as a rounded or chamfered profile, to the edge of the wafer. The opposed major surfaces of the wafer are then lapped to the desired thickness while planerizing the wafer by reducing thickness variations and improving flatness across each major surface. The opposed major surfaces are then typically etched so as to reduce the number of surface defects, before polishing at least one of the major surfaces to have the desired mirrored finish.
In order to lap a wafer, lapping machines are utilized. Lapping machines generally include a lapping carrier that defines at least one, and more commonly, a plurality of openings sized to receive respective wafers. The lapping machine also includes a pair of lapping plates disposed on opposite sides of the lapping carrier. Since the lapping carrier is slightly thinner than the wafers, the opposed surfaces of the wafer contact the lapping plates. As such, relative movement of the lapping carrier with respect to the lapping plates removes material from the opposed surfaces of the wafers, thereby lapping the wafers. In order to facilitate the lapping of the opposed surfaces of the wafers, a slurry is generally disposed between the lapping plates and the lapping carrier.
In a conventional lapping machine, multiple wafers are concurrently lapped in a batch process. Thus, the lapping carrier preferably defines a plurality of openings for receiving respective wafers. In addition, the lapping plates may be much larger than a lapping carrier such that multiple lapping carriers can be simultaneously disposed between the pair of lapping plates. In order to provide for the relative motion between the lapping plates and the lapping carriers that is necessary to lap the wafers, conventional lapping machines include an inner sun gear and an outer ring gear. Correspondingly, the lapping carriers generally include gear teeth that extend circumferentially thereabout and radially outward for engaging the inner sun gear and the outer ring gear. By appropriately driving at least one of the inner sun gear and the outer ring gear, the lapping carriers and, in turn, the wafers carried by the lapping carriers will move in a somewhat eccentric pattern between the opposed lapping plates with the wafers rotating freely within the respective openings.
Lapping carriers generally have a circular shape. Lapping carriers may have various diameters with diameters of 20″, 22″, 30″, and 32″ being relatively common. Lapping carriers are generally formed of steel and, as explained below, are typically formed of relatively hard grades of steel with hardness in the range of Rockwell C 40 to 50. The sheet steel utilized to construct lapping carriers must generally be custom fabricated since the steel must not only be hard, but the opposed surfaces of the lapping carrier must be extremely flat to facilitate the proper lapping of the wafers. In this regard, the thickness of a lapping carrier is generally subject to a very tight tolerance, such as a tolerance permitting variations in thickness of no more than +/−0.02 mm. For smaller lapping carriers, such as those lapping carriers having a diameter of 20″ or 22″, sheet steel that has been heat treated to attain a hardness ranging from Rockwell C 40 to 50, and that meets the dimensional requirements is readily available and may be purchased relatively economically. For larger lapping carriers, such as lapping carriers having a diameter of 30″ or more, however, sheet steel that meets the dimensional requirements and that has been heat treated to have the desired hardness is extremely rare due to a lack of available manufacturing sources. As the width requirement of the steel increases for larger lapping carriers having a diameter of 30″ or more, it becomes increasingly difficult for steel producers to achieve the desired thickness tolerance, and the manufacturing infrastructure for heat treating of such dimension becomes extremely rare. The lack of industry supply makes the sheet steel of the desired hardness prohibitively expensive. As such, these larger lapping carriers are generally formed of a softer grade of hard rolled steel like: SK-5 (JIS Standard), W1–8 (AISI/ASTM Standard), and 1074/1075/1086 (SAE Standard), which is more economical, but will wear more rapidly as a result of being softer.
In a typical process for fabricating a lapping carrier, openings are punched through a circular steel workpiece with the diameter of the openings being slightly larger than the diameter of the wafers such that wafers can be seated within respective openings. The edge of these openings are typically polished to facilitate rotation of the wafers within the respective openings. Additionally, the gear teeth are formed about the circumference of the circular workpiece, such as by punching or by laser cutting. For the smaller lapping carriers, since the workpiece is generally formed of a relatively hard material, the difficulty associated with forming the openings and the gear teeth may be somewhat increased.
During the lapping process, a wafer generally freely rotates within the respective opening defined by the lapping carrier in order to evenly lap the wafer as required to obtain the desired flatness. During this process, the wafer repeatedly contacts the edge of the opening. This contact between the edge of the opening and a wafer causes the edge of the opening to gradually degrade or erode. This degradation of the opening may cause the edge of the opening to become grooved and roughened, as opposed to a flat and smooth edge as desired. The degraded edge of an opening impedes rotation of the wafer, thus causing the wafer to be lapped more unevenly and decreasing the flatness of the resulting wafer. The degraded edge of the opening may also damage the edge of the wafer, thereby increasing the possibility that the edge of the wafer will chip. As the edge of the openings defined by the lapping carrier further erodes, wafers may actually become dislodged from the respective openings during lapping operations. In this instance, the lapping operations would crash and the lapping machine would need to be halted, disassembled, cleaned and potentially the lapping carrier would need to be replaced, prior to being returned to service. Since the rate at which the edge of the opening degrades is based, at least in part, upon the hardness of the lapping carrier, larger lapping carriers that are generally formed of softer grades of steel typically experience erosion of the edge of the openings at a quicker rate than that experienced by smaller lapping carriers that are generally formed of harder grades of steel
In order to maintain the relatively free rotation of wafers within the openings defined by a lapping carrier and to avoid the deleterious effects occasioned by the degradation of the edge of the openings defined by the lapping carrier, lapping carriers are periodically replaced. In this regard, degradation of the edge of the openings of a lapping carrier is the most common reason for replacing a lapping carrier. However, lapping carriers are also replaced because of undesirable thinning of the lapping carrier. In this regard, the lapping process in which the opposed surfaces of the wafers are lapped by a polishing slurry also removes material from the opposed surfaces of the lapping carrier. While the lapping carriers are designed to be somewhat thinner than the desired thickness of the wafers, such as about 5 microns to about 100 microns thinner, a lapping carrier is no longer usable if the lapping carrier becomes substantially thinner than the wafers. As will be apparent, the replacement of a lapping carrier increases the capital costs associated with the lapping process since lapping carriers are relatively expensive, while slowing the overall fabrication process that must be temporarily halted in order to replace the lapping carrier.
In order to reduce the damage on the edge of the wafer caused by the degradation of the edge of the openings defined by a lapping carrier, lapping carriers have been designed having injection molded plastic rings or manually applied plastic inserts that are fitted to the edge of the openings. See, for example, U.S. Pat. No. 6,454,635 Zhang, et. al, U.S. Pat. No. 6,514,424 to Guido Wenski et al. and U.S. Pat. No. 5,914,053 to Masumura, et al. The plastic rings and inserts create a smooth and buffered contact surface within the opening of lapping carrier, which reduce the effect of the impact force generated between the wafer and the edge of the opening of the carrier during lapping. This helps reduce the possibility of wafer chipping, while promoting free rotation of the wafer. However, since of the plastic rings and inserts are softer than the steel, the rate of erosion of the edge of the openings defined by the lapping carrier will be faster than that of steel. As such, lapping carriers with injection molded plastic rings must be replaced at shorter service life than standard carrier, while lapping carriers with manually applied plastic inserts must be taken off-line more frequently to have the insert reapplied. With respect to at least the larger lapping carriers that are generally formed of a softer grade of steel, the lapping carriers may also bend more easily during use or general handling, thereby weakening the bond between the plastic ring or insert and the lapping carrier in instances in which the plastic ring or insert is adhered to the edge of a respective opening.
It would therefore be desirable to provide a lapping carrier constructed from an economical and widely available material that can be cryogenically enhanced to achieve a longer useful life. In this regard, it would be desirable to provide a lapping carrier having openings with edges that are not degraded as quickly and having a thickness that does not decrease as rapidly during lapping in comparison to conventional lapping carriers. Furthermore, with respect to lapping carriers with manually applied plastic inserts, it would be desirable to provide a lapping carrier with a reduced rate of wear on the steel carrier body. This allows for repeated application of manually applied plastic inserts, thus further extending the service life of that carrier, and reducing overall capital cost. Lastly, it would be desirable to provide a lapping carrier whose hardness is not constrained by the manufacturing capability of steel manufacturer, but can be substantially manipulated during the manufacturing of the lapping carrier to achieve a martensite crystalline structure of at least 70%, and more advantageously at least 90%, and even more advantageously at least about 99%. This will provide greater flexibility for the procurement of raw material in terms of price, quality, availability, and timeliness of delivery.